| Summary: | 碩士 === 義守大學 === 材料科學與工程學系碩士班 === 95 === Air plasma sprayed thermal barrier coatings (TBCs) are widely used to protect components of aero- and land-based gas turbines engines against deterioration at high temperature. However, the existence of interconnected porosity and segmented cracks in TBCs affects the mechanical properties and decreases lifetime. Air plasma sprayed yttria partially stabilized zirconia thermal barrier coatings and laser re-melting is combined to improve properties of TBCs through melting quickly and then rapid solidification process. Recently, the research on new materials progresses rapidly. Laser has been applied to improve the surface properties, such as wear-resisting, hardness, and permeability have all been affected by surface heat treat of laser. The study probes into the thermal barriers coating phenomenon of yttria-partially stabilized zirconia in surface heat treat with YAG laser. The purposes are to produce new surface organization and structure in the rapid solidification process by high-temperature laser melting surface ceramics, and then determine the surface mechanical properties. When the surface layer of material starts solidifying, thermal stresses will be produced and the makes crack to release the thermal stresses. Observing surface forms and crack phenomenon through laser heating up by Scanning Electron Microscope (SEM), and then analyzing by X-ray diffraction (XRD).The experiential results show that different laser parameters produce several microstructural features including homogeneous, cellular structure and flatness. In addition, it displays melted pool phenomenon that is accompanied by concentric ring and cracks because of rapid solidification process. Increasing laser fluence increases the width of cracks in the free surface and cellular structure becomes bar structure close to outside of concentric ring. The cross sections become denser, change from lamellar to dendritic columnar and generate vertical cracks. For hardness and fracture toughness are positive proportional to laser fluence. On the contrary, crack lengths are inversely proportional to laser fluence. Concerning X-ray diffraction the intensity of peaks and full-width at the half-maximum decrease with increasing laser fluence. The growth of grain sizes brings about a great fracture toughness. In summary, slow scanning speed, short focal distance and high laser fluence made a higher fracture toughness.
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